1. Technical Field
The present invention relates to a DC offset canceller used in, for example, a current reuse type differential amplifier.
2. Description of the Related Art
In recent years, networks such as a wireless personal area network (WPAN) and a wireless sensor network have garnered much attention. To create such networks, compact and low power consumption wireless terminals are necessary. Since a large number of differential amplifiers are incorporated into a wireless terminal, it is significantly important to reduce power consumption of the differential amplifiers. To reduce the power consumption of a differential amplifier, it is important for the differential amplifier to support a low-voltage power supply and provide a high amplification factor even when the consumption current is small. For example, a current reuse type amplifier described in IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. 55, No. 7, JULY 2007 meets such requirements.
The direct current component (hereinafter simply referred to as a “DC component”) of the output voltage of a differential amplifier typified by a current reuse type differential amplifier varies from differential amplifier to differential amplifier due to a difference in property between FETs that form a differential pair. Such a difference in a DC component is referred to as a “DC offset”. The DC offset causes a decrease in the linearity and the amplification factor, which are the indices of the performance of a differential amplifier. In addition, the DC offset promotes degradation of the system performance of, for example, a wireless terminal having the differential amplifier mounted therein.
Some differential amplifiers include a DC offset canceller that cancels out the DC offset. For example, Japanese Unexamined Patent Application Publication Nos. 2007-243636 and 2004-040157 describe such differential amplifiers. The DC offset canceller described in Japanese Unexamined Patent Application Publication No. 2007-243636 compares the output voltage of a differential amplifier with a reference voltage. Thereafter, an AC component contained in the comparison result is removed, and a control signal based on the result is output to the differential amplifier so that the output voltage of the differential amplifier is the same as the reference voltage. To remove the AC component contained in the comparison result between the output voltage of the differential amplifier and the reference voltage, a lowpass filter (LPF) is mainly used. In contrast, the DC offset canceller described in Japanese Unexamined Patent Application Publication No. 2004-040157 extracts the DC component of each of the outputs of the differential amplifiers and removes the DC offset from the extracted DC components.